1. Field of the Invention
The present invention relates generally to radiation therapy devices, and more particularly, to a removable electron multileaf collimator for use in a radiation therapy device.
2. Discussion of Related Art
Conventional radiation therapy typically involves directing a radiation beam at a tumor in a patient to deliver a predetermined dose of therapeutic radiation to the tumor according to an established treatment plan. This is typically accomplished using a radiation therapy device such as the device described in U.S. Pat. No. 5,668,847 issued Sep. 16, 1997 to Hernandez, the contents of which are incorporated herein for all purposes.
The radiotherapy treatment of tumors involves three-dimensional treatment volumes which typically include segments of normal, healthy tissue and organs. Healthy tissue and organs are often in the treatment path of the radiation beam. This complicates treatment, because the healthy tissue and organs must be taken into account when delivering a dose of radiation to the tumor. While there is a need to minimize damage to healthy tissue and organs, there is an equally important need to ensure that the tumor receives an adequately high dose of radiation. Cure rates for many tumors are a sensitive function of the dose they receive. Therefore, it is important to closely match the radiation beam's shape and effects with the shape and volume of the tumor being treated.
Either primary photon or primary electron beams may be used in radiation therapy. Currently, clinical practice requires substantial manual intervention to use conformal electron treatment. Conformal photon fields typically are shaped using one or more collimating devices positioned between the source and the treatment area. Many of these photon beam collimating devices (multi-leaf collimators or MLCs) are positioned automatically to deliver a desired photon field shape to a treatment area on a patient. Little manual intervention is required to administer photon radiation therapy. A new type of therapy is also emerging, which involves using both photon beams and electron beams in the same treatment, here called “Mixed Beam Radiotherapy”. To be practical, Mixed Beam Radiotherapy requires advances in electron delivery, such as an automatic collimating device designed explicitly to shape electrons such as disclosed in U.S. patent application Ser. No. 09/909,513, filed on Jul. 20, 2001, the entire contents of which are incorporated herein by reference. The photon MLC and the new electron MLC need to be coordinated in an optimal way.
FIG. 1 schematically shows a radiation therapy machine 10 that includes a gantry 12 which can be swiveled around a horizontal axis of rotation 14 in the course of a therapeutic treatment. A treatment head 16 is fastened to a projection of the gantry 12. A linear accelerator (not shown) is located inside gantry 12 to generate the high energy radiation required for the therapy. The axis of the radiation bundle emitted from the linear accelerator and the gantry 12 is designated by beam path 18. Electron, photon or any other detectable radiation can be used for the therapy.
During a course of treatment, the radiation beam is trained on treatment zone 20 of an object 22, for example, a patient who is to be treated and whose tumor lies at the isocenter of the gantry rotation. Several beam shaping devices are used to shape radiation beams directed toward the treatment zone 20. For example, a multileaf photon collimator and a multileaf electron collimator can be arranged to shape the radiation beams. Each of these collimators may be separately controlled and positioned to shape beams directed at treatment zone 20.
For example, when the electron beam source is used, the multileaf photon collimator may be fully retracted and the multileaf electron collimator is designed specifically to stop the primary electrons. However, a few electrons in the beam have bremsstrahlung radiation interactions with high atomic number materials in the head of the accelerator that result in a low percentage photon component (3-5%) to the beam that are not stopped by the electron collimator. This component may be considered “leakage” since it may not be noticeably attenuated by the multileaf electron collimator and will cause an unmodulated background component to the distribution. This is not a significant problem for single electron fields, in fact it may be considered useful since it is possible to get an image of the field from this component with an extremely sensitive portal imaging system such as described in U.S. patent application Ser. No. 09/910,526, the entire contents of which are incorporated herein by reference. If electron modulation is introduced, however, the number of segments or individual fields in an Intensity Modulated Radiation Therapy (IMRT) sequence is increased. A significant increase to the integral dose may result if many segments are used because the photon leakage through the multileaf electron collimator is summed from each segment.
One possible solution is to make the leaves of the multileaf electron collimator thick enough to attenuate the photon component, but this increases the size and weight of the accessory considerably. A second possible solution is to use the multileaf photon collimator in such a way that it acts as a “back up” attenuator. This technique will nearly eliminate the photon component outside the field, but the effect of the multileaf photon collimator 116a and jaws 116b on the electron field itself must be considered.
Some of the electrons that contribute to the field at the patient plane originate from scattering off of secondary “sources” along the beamline, such as the scattering foils and the air column just outside of the beam. Thus, the multileaf photon collimator and the jaws block part of the field if they are fitted to the same size and shape as the field defined by the multileaf electron collimator alone. The result is a broadened penumbra and reduced output due to the scattered electrons.
Accordingly, when bremsstrahlung leakage is generated through a multileaf electron collimator, it is desirable to reduce dosage applied to a patient while providing as clean a beam as possible for the mixed beam treatment. The ideal margin for the photon multileaf collimator for each electron field is a compromise between these two competing interests. In general the margin is a function of the secondary electron energy of the secondary electrons generated from the scattered primary electrons.